1. Field of the Invention
The present invention relates to laser systems. In particular, the present invention relates to implementing active loads in the discharge circuitry design of gas discharge lasers to provide stabilization of high repetition rate gas discharge laser systems.
2. Description of the Related Art
Pulse gas discharge lasers, emitting in the deep ultraviolet (DUV) and vacuum ultraviolet (VUV) region are widely used in various industrial applications such as microlithography, photoablation, and micro-machining, among others. For microlithographic applications, currently used systems include line narrowed excimer lasers, such as ArF (193 nm) lasers and KrF (248 nm) lasers, as well as molecular fluorine (F2) lasers emitting at 157 nm, which are efficient and exhibit high energy stability at high repetition rates, for example, at 1-2 KHz or more. FIG. 1 illustrates a schematic arrangement of a pulsed gas discharge electrical circuit of a typical gas discharge laser system. As shown, a pair of discharge electrodes 101, 102 is coupled to a discharge circuit which includes a peaking capacitance Cp and an inherent inductance Ld between the peaking capacitance Cp and the discharge electrode 101. Such discharge electrical circuitry may be found in current gas discharge lasers such as excimer lasers and molecular fluorine laser systems.
Referring to FIG. 1, the area between the discharge electrodes 101, 102 defines a region referred to as a gas discharge region 103. As can be seen, the pair of elongated discharge electrodes 101, 102 of the gas discharge laser, one of which (in this case, the discharge electrode 102) may be connected to a ground or reference potential, are separated by the gas discharge region 103 which is filled with a high pressure laser gas. Moreover, the discharge electrode 101 is connected to the output of the high voltage pulsed generator which is capable of providing fast and powerful charging of the peaking capacitor Cp up to the electrical breakdown voltage of the gas discharge gap. Powerful pulsed source(s) or preionization units for providing ultraviolet light in a spark or corona discharge provide an initial preionization of the gas mixture in the discharge region, and are typically positioned in the vicinity of the gas discharge region 103 between the discharge electrodes 101, 102. The preionizer(s) provide an initial ionization, or preionization, of the laser gas during the charging of the peaking capacitance Cp which receives an electrical pulse initially provided by the charging of a main storage capacitor by a high voltage (HV) power supply when the main storage capacitor is discharged through a switch such as a thyratron or a solid state switch.
Referring back to FIG. 1, in such gas laser systems the HV electrical circuitry which is used for the excitation of the gas discharge in the pulsed gas laser systems may be schematically sub-divided into two parts. The first part of the HV electrical circuitry may include the peaking capacitance Cp which is configured to store electrical energy, and used during the gas breakdown phase. The second part of the HV electrical circuitry may include the HV pulsed power generator which is used for the fast and efficient charging of the peaking capacitor Cp up to the breakdown voltage of the gas. In particular, the HV pulsed power generator may include a suitable HV pulsed device such as a gas filled thyratron, or a solid state switch such as a thyristor or an IGBT).
Additional information may be found in R. S. Taylor, K. E. Leopold, Applied Physics, B59 (1994) 479; U.S. Pat. Nos. 6,020,723, 6,005,880, 5,729,562, 5,914,974, 5,936,988, 6,198,761, 5,940,421, and 5,982,800, and pending U.S. patent applications Ser. Nos. 09/649,595, and 09/453,670, the disclosures of each of which are expressly incorporated herein by reference for all purposes.
A problem encountered with typical pulsed electrical gas discharges in strongly electronegative gas mixtures (i.e., containing a halogen component) at elevated pressures (for example, several bars) is a certain degree of instability. The short phase of the uniform glow discharge, usually less than 100 nanoseconds, corresponding to the pumping of the laser medium, is typically terminated by rapidly developing streamers. The streamers themselves are temporally inconsistent which leads to the discharge instabilities. In addition, the existence of streamers at the ends of discharges produces excessive wear on the electrodes. In view of these problems caused by streamers, it is desired to suppress them. It is therefore desired to have a gas discharge laser including a discharge circuit wherein the main input of the energy into the gas discharge is quickly realized, or that provides very short, intense electrical pulses to the main discharge load, and is terminated without extended and inconsistent streamers reducing the discharge stability from pulse to pulse and without excessively wearing the discharge electrodes.
In view of the foregoing, a discharge circuit for a pulsed gas laser system in accordance with one embodiment of the present invention includes a pair of spaced-apart electrodes defining a discharge region as a main load, a capacitance coupled to one of the pair of electrodes for providing electrical pulses to the electrodes, and an additional load electrically coupled between the capacitance Cp and one of the discharge electrodes.
The additional load may include one or more resistors, a resistor array, a resistor or resistor array coupled with a variable inductance and/or a saturable inductance, or another dissipative electrical component for dissipating electrically energy between the main load and the capacitance to facilitate termination of electrical discharges between the electrodes and in turn suppress the formation or influence of streamers. The additional load may be coupled in series or in parallel with the capacitance and the main load, and a portion of the additional load may be coupled in series and a portion may be coupled in parallel with the capacitance and the main load. Any series connected portion of the additional load may be coupled to a high voltage or grounded main electrode. The resistor or resistors may have a value comparable to a wave impedance of the discharge circuit. Alternatively, the resistor or resistors may have a value comparable to an active impedance of the gas discharge during a maximum discharge current phase. The additional load is preferably a passive resistance, and may alternatively have an active feature such as a voltage dependence or a temperature dependence.
The circuit may further include a cooling unit, wherein the additional load is provided in the cooling unit. The cooling unit may be provided within a pulsed power module of a laser system which contains electrical components of the discharge circuit particularly susceptible to heating effects. The cooling unit may include one of an air fan and an encapsulated volume with circulating isolating fluid.
One or more preionization units are preferably also provided for ionizing the laser gas within the discharge region between the pair of main electrodes during the charging of the capacitance just prior to discharging through the electrodes.
The capacitance may include a series of peaking capacitors, and may include a series of sustaining capacitors. The sustaining capacitors would be coupled to the electrodes by a different inductance than the peaking capacitors, and would be otherwise preferably coupled within the discharge circuit similarly to the peaking capacitors defining the capacitance coupled to the main electrodes. The pair of electrodes, the capacitance and the additional load may form a series configuration such as an electrical loop, or the additional load may be coupled to the electrodes in parallel with the capacitance. The circuit further includes a power generator configured to provide power to the capacitance for charging the capacitance. The power generator may include a high voltage pulsed power generator. The power generator is preferably connected to a main storage capacitor which is charged during a charging cycle. A solid state switch is used for discharging the main storage capacitor to the rest of the discharge circuit which preferably includes one or more pulse compression stages before the peaking capacitance connected to the main load. A transformer and/or voltage doubling circuit may also be coupled between the main storage capacitor and the pulse compression stages. A processor is connected in a pulse energy or energy dose control feedback loop with a detector for providing charging voltage values to which the power supply charges the main storage capacitor between discharges of electrical pulses.
The circuit may further include a ground terminal coupled to the capacitance.
A discharge circuit in accordance with another embodiment of the present invention includes a pair of discharge electrodes, an area between the pair of electrodes defining a gas discharge area, a peaking capacitance coupled to one of the pair of discharge electrodes, the peaking capacitance configured to store charge, an additional load including a resistor, resistor array for low inductivity, or a resistance coupled with a saturable or variable inductance is coupled between the discharge electrodes and the peaking capacitance, either in series or in parallel, and a ground or reference voltage terminal is further preferably coupled to the other terminal of the peaking capacitance and the main load or discharge electrodes, where the pair of discharge electrodes, the peaking capacitor and the resistor form an electrical loop. That is, a first electrode of the pair of main electrodes is preferably coupled to the peaking capacitance, wherein the additional load is coupled either in series between the peaking capacitance and the first main electrode, and a second main electrode is connected to a ground or reference voltage along with a ground or reference terminal of the peaking capacitance. When the additional load is connected in parallel with the peaking capacitance, the additional load is also preferably connected to the ground or reference voltage.
The circuit may further include a cooling unit for cooling the additional load or resistance.
The gas discharge area may be filled with a high pressure laser gas, e.g., at 2-7 bar, and preferably around 3-5 bar. The gas mixture may include molecular fluorine and an active rare gas such as krypton or argon, of a KrF or ArF laser, respectively, while the gas mixture may be pressurized with a buffer gas of neon and/or helium. The laser active gas may solely include molecular fluorine such as for a molecular fluorine laser, wherein the buffer gas may include neon and/or helium or a combination thereof.
A resistor or a resistor array may be connected between the peaking capacitor and high voltage main electrode, while the other discharge electrode is connected to ground or a reference voltage.
A discharge circuit for use in a laser system in accordance with yet another embodiment of the present invention includes a pair of discharge electrodes, wherein an area between the pair of electrodes defines a gas discharge area. A first peaking capacitance is coupled directly to the pair of electrodes. A second peaking capacitance is also coupled to the pair of electrodes, wherein a resistance or an otherwise additional load is coupled, either in series or in parallel, between the second peaking capacitance and the discharge electrodes. A ground or reference terminal is preferably coupled to the first and second peaking capacitors and a ground electrode of the pair of main discharge electrodes, wherein the pair of discharge electrodes, the second peaking capacitances and the resistance or additional load may form a series electrical loop or parallel electrical combination, while the first peaking capacitance and discharge electrodes form another electrical loop. The additional load may include a resistor, a resistor array, a resistor combined with a variable or saturable or saturable inductance, or other means for dissipating electrical energy between the discharge electrodes and the second peaking capacitance.
The circuit may include a cooling unit for cooling the resistor.
The circuit may include a high voltage pulsed generator to provide power to the first and second peaking capacitors, and the gas discharge area may include a high pressure laser gas.
A method of providing a discharge circuit for a pulsed gas laser system in accordance with still a further embodiment of the present invention includes providing a pair of electrodes, coupling a capacitance to a first electrode of the pair of electrodes, the capacitance configured to store charge, and coupling an additional load between the capacitance and the first electrode.
A further method includes charging a main storage capacitor of a pulsed gas discharge excitation laser system, discharging the main storage capacitor through a pulse compression circuit to a peaking capacitance as an electrical pulse, and dissipating the energy of the electrical pulse through a main load and an additional load, the main load including a discharge region filled with a gas mixture and the additional load including a resistor, resistor array or a resistance coupled with a variable or saturable inductance, and wherein the additional load is coupled either in series between the peaking capacitance and the main load or in parallel with the peaking capacitance.
According to either of the above methods, the load may include a resistor, and the resistor may have a value comparable to a wave impedance of said discharge circuit, or a value comparable to an active impedance of the gas discharge during a maximum discharge current phase.
The method may further include the step of providing cooling the additional load, where the step of cooling may include the step of providing either an air fan or an encapsulated volume with circulating oil.
The method may further include the step of defining a volume between the pair of electrodes as a gas discharge area having a width that allows a clearing ratio of the laser gas to be sufficient in view of the repetition rate of the laser, which may be 2 kHz, 4 kHz or more.
The method may further include the step of providing ionization of a laser gas in the gas discharge area during the charging of the capacitance.
The pair of electrodes, the capacitance and the load may form an electrical loop.
A method of providing a discharge circuit in accordance with yet still another embodiment of the present invention includes defining an area between a pair of discharge electrodes as a gas discharge area, coupling a peaking capacitance to the pair of discharge electrodes, coupling an additional load between one of the discharge electrodes and the peaking capacitance, and coupling a ground or reference voltage terminal to one of the discharge electrodes which couples either directly to a ground terminal of the peaking capacitance or to the peaking capacitance through the additional load, where the pair of discharge electrodes, the peaking capacitance and the additional load form a series or parallel electrical combination.
A method of providing a discharge circuit for use in a laser system in accordance with yet a further embodiment of the present invention includes providing a pair of discharge electrodes, an region between the pair of electrodes defining a gas discharge region, coupling a first peaking capacitance to the pair of electrodes, and coupling a second peaking capacitance to the pair of electrodes, and coupling a resistance or an otherwise additional load between the second peaking capacitance and the discharge electrodes, and coupling one or the discharge electrodes to ground, along with coupling at least one of the first and second capacitances also to ground, wherein if the additional load is coupled between the second peaking capacitance and the ground discharge electrode, then the second capacitance is coupled to ground through the additional load, and if the additional load is coupled to the high voltage discharge electrode, the the second capacitance is coupled directly to ground, and the first capacitance is coupled directly to ground in either case.
These and other features and advantages of the present invention will be understood upon consideration of the following detailed description of the invention and the accompanying drawings.